Valorization of Fruit Co-Product Flours for Human Nutrition: Challenges, Applications, and Perspectives

: The fruit agro-industry is one of the sectors that stand out both in production and in the volume of losses along the supply chains, which has generated a strong concern from the nutritional, economic, social, and environmental points of view. This study is aimed at understanding the updated scenario of the conversion of fruit residues into value-added co-products, its main challenges, applications, and perspectives. For this, a literature review was conducted through Scielo, PubMed, Google Scholar, and ScienceDirect databases. The advanced search covered the period from 2018 to 2022. The evaluation of the articles showed that the drying process is an important step to obtain ﬂours from fruit co-products with characteristics that can provide longer shelf life, practicality, and versatility of use, demonstrating great potential for inclusion in various food preparations; although difﬁculties persist around the technological characteristics of this raw material, they can confer a nutritional increase, in addition to the possibility of additional health beneﬁts due to the presence of bioactive compounds and ﬁbers inherent in these products. Indeed, although there is a long way to go in studies with co-products derived from residual fractions of fruits, strategies such as these contribute to the better management of losses along the agri-food chains while providing greater food and nutrition security for the global population on the path to sustainability.


Introduction
In a world that is in constant transformation, all issues related to the harmonious maintenance of life must go through sustainability.In all dimensions, sustainable actions should guide attitudes, guidelines, and policies capable of managing natural resources to ensure conscious and responsible use.The food sector has been the major focus of research in the last decade and advances in legislation, regulatory frameworks, and guidelines for waste management indicate a scenario of valorization of co-products as the key point for sustainability in the food sector [1].About one-third of the food produced worldwide is lost or wasted, and it is estimated that fruits and vegetables represent 40 to 50% of this total [2].Because of this situation, this issue has gained strength in recent years, going beyond the analysis of specific sectors and gaining a transversal and global approach, since, in addition to contributing to the practice of unsustainable food systems, these losses put pressure on natural capital and aggravate the situation of hunger and poverty worldwide.
The 2030 Agenda contemplates, in its Sustainable Development Goal (SDG) number 12, the proposal to ensure sustainable production and consumption patterns, with the goal of reducing food losses and waste along the production and supply chains by 2030 [3].According to the Food and Agriculture Organization of the United Nations (FAO), "losses" refer to the decrease in the mass of edible food in all parts of the supply chains intended for

Materials and Methods
This study was based on an updated literature review on the use of fruit co-products (shells and seeds) to obtain flours with applications in human nutrition.To this end, an advanced search was carried out in the Scientific Electronic Library Online (Scielo), PubMed, Google Scholar, and ScienceDirect databases, covering the period from 2018 to 2022.The analysis sought to collect data on two questions: (i) drying conditions applied to obtain flours from waste fractions of the fruit agro-industrial chain (peels and seeds); and (ii) Perspectives on the use of fruit co-product flours in the elaboration of food products.The descriptors: "drying", "co-products", "fruits", "shells", and "seeds" were used, alternating between the Boolean operators OR and AND.In addition, the filters "Food bioscience", "Food chemistry", "Food science" and "Food science and innovation" were used to better direct the search.Only original studies were used, excluding review studies.The first stage of the analysis was guided by the reading of titles and keywords.The articles belonging to the scope of the research were excluded from duplicates.This was followed by reading the abstracts and, when necessary, the methodology.At this stage, the following exclusion criteria were used: (i) flours from co-products obtained from mixed pomace, stalks, or whole fruits derived from losses in the supply chain; (ii) studies with incomplete information on processing methods for obtaining flours; (iii) mixed flours with added insects or crustaceans; (iv) flours obtained without the drying step; and (v) flours elaborated for the purpose of inclusion in animal nutrition.The articles resulting from this step were read in full to choose those eligible for this study.The search scheme is detailed in Figure 1.
stage, the following exclusion criteria were used: (i) flours from co-products obtained from mixed pomace, stalks, or whole fruits derived from losses in the supply chain; (ii) studies with incomplete information on processing methods for obtaining flours; (iii) mixed flours with added insects or crustaceans; (iv) flours obtained without the drying step; and (v) flours elaborated for the purpose of inclusion in animal nutrition.The articles resulting from this step were read in full to choose those eligible for this study.The search scheme is detailed in Figure 1.

The Importance of Converting Fruit Agro-Industry Waste into Co-Products
The fruit agro-industry is one of the most relevant segments of the economy; however, it registers one of the highest percentages of losses among the productive sectors, which in addition to the economic factors involved, also have repercussions on the environment and on the food security of the population [14,15].This is partly due to the high volume of waste resulting from the processing of fruit products, such as juices, jellies, and dehydrated products.This waste is often improperly disposed of in landfills or waterways, or is even burned, causing various environmental damages and health-related risks and becoming a challenge in food waste management [16,17].
Although some countries practice composting and anaerobic digestion or even use part of these fractions for animal feed, a considerable portion of what is produced glob-

The Importance of Converting Fruit Agro-Industry Waste into Co-Products
The fruit agro-industry is one of the most relevant segments of the economy; however, it registers one of the highest percentages of losses among the productive sectors, which in addition to the economic factors involved, also have repercussions on the environment and on the food security of the population [14,15].This is partly due to the high volume of waste resulting from the processing of fruit products, such as juices, jellies, and dehydrated products.This waste is often improperly disposed of in landfills or waterways, or is even burned, causing various environmental damages and health-related risks and becoming a challenge in food waste management [16,17].
Although some countries practice composting and anaerobic digestion or even use part of these fractions for animal feed, a considerable portion of what is produced globally is discarded.The high biodegradability of this waste, associated with its high moisture content, favors high microbial loads.Moreover, the anaerobic biological degradation of organic matter is considered the third largest anthropogenic source of atmospheric methane emissions, considered one of the main greenhouse gases [16,18], which justifies the widespread interest in the search for more sustainable applications.This problem within the fruit chains has highlighted the urgency of implementing elements that bring about change to strengthen the production chains through practices that contribute to regional socioeconomic development combined with strategies that intensify food and nutritional security measures, which brings to light the need for greater conversion of these fractions into products that, with the use of adequate processing, guarantee quality and safety so that they can be used in human nutrition [14].
It is estimated that approximately 14% of food produced worldwide is lost between production and post-harvest.In developed countries, this waste is mainly concentrated at the final consumer, unlike in underdeveloped countries, where it occurs throughout the supply chain [19].In order to have a better notion of the scenario of the fruit processing industry, 30 to 50% of fractions consisting of peels or seeds are generated; for products that are mostly discarded and even in the face of their potential, the use is still considered low [20], stimulating the underutilization of food, a global problem that contributes to the food and nutrition insecurity experienced by a significant portion of the global population [21,22].
The Food and Agriculture Organization of the United Nations (FAO) has noted that along the stages of the entire food chain, losses can amount to approximately half of what is produced [23].This amount would be enough to feed at least 1 billion people in a state of malnutrition [24].This is a worrying fact in a panorama of contrast between losses and hunger that plagues the world, which requires urgent measures.It is in the context of the circular economy that the valorization of co-products promises to be an efficient option in the medium and long term for the reuse and recovery of natural resources through the reintroduction of raw materials to obtain new products with health benefits through appropriate technologies [25,26].
By encouraging the circular economy, it is possible to convert the system of extraction, exploitation, and consumption to a system of restoration and regeneration of natural resources, avoiding the unnecessary generation of waste, acting to reduce social inequality caused by poverty and hunger, and obtaining more affordable food in accordance with the applicable laws of each country [16].It is also worth noting that, regardless of the origin of the problem, the poor management of the fruit chain waste endorses losses and impacts the availability of food, with negative repercussions on the population's food security, which requires reflection; tackling this problem efficiently, sustainably, and in an integrated manner requires the optimization of natural and financial resources [27].
It is important to note that, although waste is considered a negative result of a process, co-products can have a positive impact on the food supply chain, provided they are properly managed [28].It is estimated that a reduction in agro-industrial waste of between 30 and 50% could increase the available food supply by at least 15% with the appropriate use of these fractions [29].
Therefore, the re-signification and valorization of all the material generated along the fruit agro-industrial chain require an understanding of the technological, nutritional, and safety aspects.The use of co-products from fruit processing chains is a promising strategy to meet the demands for nutritious and sustainable agro-ecological food.The incorporation of co-products by the food industry as non-conventional ingredients and in the development of new products is part of an important framework for strengthening environmental sustainability, encouraging practices that value regional cultures, waste management, and product diversification for an increasingly demanding consumer market.

Flours from Unconventional Parts: Drying Challenges as an Alternative in the Treatment of Fruit By-Products
Seeds and peels are classified as inedible parts of the fruit when processed or consumed in natura, although numerous compounds and functional characteristics that add value to these fractions are widely reported in the scientific literature [26,30].For this reason, the use of these co-products as primary or secondary ingredients in human food as a source of nutrients, dietary fibers, and bioactive compounds has aroused interest in the food industry [31].There are several residual fractions from fruit processing that can be used in human food [26]; but for this, an essential step is to evaluate the composition of the co-products since this is influenced by factors, i.e., stage of ripeness, seasonality, species, harvest season, among others.In addition, another important point is the choice of the most suitable process for the objectives to preserve the compounds of interest without causing harm to the consumer's health.
One of the great challenges of using fresh by-products is their high perishability.In general, fruit by-products have between 60% and 80% moisture content, which is a negative factor for microbiological safety and product quality.This characteristic can also act as a limiting factor in the strategy of spreading the use of co-products, as it increases transportation costs and limits the distribution radius.For this reason, drying has become an indispensable stage, not only for the quality of the product but also for increasing the scope of using co-products [32].Drying operations are the main means of obtaining products with a longer shelf life, providing the ability to be stored away from refrigeration, providing a reduction in losses during harvest periods, and providing a reduction in the cost of treating the resulting waste [33].In addition, heat treatments can also inactivate lipid-modifying enzymes and, therefore, prevent lipid oxidation [34].Drying can lead to some loss of bioactivity due to the thermal degradation of the more fragile molecules or interfere with the extraction procedure of other desirable components [35].It is also important to clarify that drying can have deleterious effects on the physical, chemical, nutritional, sensory, and functional properties of food matrices [36] and, therefore, each matrix must be evaluated according to its peculiarities and a careful study is required to decide on the method for choosing the drying conditions.Studies report that the use of different drying equipment and temperatures can influence the compounds related to flavor, color, and retention of phytochemicals, which makes choosing the best method one of the main challenges at this stage [37,38].
Santos et al. [39] evaluated various studies covering different drying methods and pointed out that drying below 65 • C preserves antioxidant activity and minimizes losses of polyphenols, tannins, anthocyanins, and proteins.Drying in air circulation ovens at temperatures of approximately 50-60 • C generates fewer changes in the profile of dietary fibers and pectic substances.The use of higher temperatures (70-90 • C) results in products with lower water retention capacity and fat adsorption capacity.However, it is important to note that factors such as the speed of the airflow and the initial humidity, among others influence the results.Therefore, the best method must comply with criteria that enables the best cost-benefit ratio to be achieved, both in economic terms and in terms of the quality of the product, which requires a holistic approach when choosing the method and equipment [37,39].In view of this, it is important to note that defining the type and conditions of drying to mitigate the loss of nutrients and preserve the nutritional value and characteristics of the product is a fundamental stage in operational planning [33,40].Table 1 shows the application of different conditions for obtaining shell and seed flours in which drying was used as a stage in the obtaining process.The products obtained can result in different forms, such as powders, flakes, and granules, which depend, for example, on the drying system used in the process, the granulometry obtained through milling, the manufacturer's requirement, or specific legislation [40,58].The resulting functional properties depend on the food matrix, the chemical and physical structure of the polysaccharides and proteins present in the fractions used (shells or seeds), or the processes used for grinding, drying, heating, extruding, or cooking, among others [59].
From the studies evaluated in Table 1, it can be seen that even when using the same raw material, large variations can be observed in the time-temperature binomial; these vary according to the type of fraction or equipment used for the process, which reinforces the need to establish protocols capable of guaranteeing the use of more economical and ecological methodologies with less expenditure of resources and ensuring the lowest possible loss of nutrients during processing.Of the studies evaluated, approximately 88% use conventional methodologies.In general, conventional drying technologies are carried out using hot air, which requires high energy consumption and contributes to greenhouse gas emissions, making it clear that there is a need to look for cheaper, more efficient, and sustainable technologies [36,60].However, the use of more modern technologies is not yet a reality applicable to many countries, especially in developing ones; so, it is essential to carry out a careful analysis of the methodologies applied to maintain a balance between the quality and safety of dried foods and the economic and environmental costs of the systems used [61].Analytical studies for process design and optimization should be implemented to standardize methodologies on an industrial scale that, even with the use of conventional methods, are able to maintain the sustainability of the process.
Given this panorama of obtaining products within viable planning, there is an incentive to search for alternatives to insert flours of fruit co-products in preparations widely consumed by the population as an alternative to value and disseminate the use of these ingredients, obtaining preparations with multifunctional characteristics and bringing health benefits, which seems to be a great strategy with a focus on more ecological and conscious actions [62,63].

Flours Derived from Fruit Co-Products: Applications and Perspectives in Human Nutrition
For the food industry, it is important that new ingredients are constantly available on the market, especially when their inclusion can add value to the product at a low cost, which makes the products more accessible and competitive.However, there is a need for these new products to meet the expectations related to sensory and technological characteristics already known by consumers, such as aspects related to color, aromas, and textures.Thus, there is an interest in a better understanding of how it is possible to include ingredients, obtained from non-conventional sources, in traditional products with market potential while offering quality products.In this sense, research has sought to understand how this type of ingredient can be used and the advantages of adding it to traditional product formulations.Some of these studies are reported in Table 2.
Table 2. Studies with the application of flours obtained from fruit co-products in the development of products for human consumption.

Study Results References
Fortification of yogurts with passion fruit peel and seed flour A = Nutritional, technological and sensory viability.D = Not reported in the study [64] Cereal bars produced with banana peel flour A = Viability for new product development; sustainable option for different niche markets.D = Not reported in the study [65] Use of watermelon seed flour as a protein supplement in cookies A = Improved nutritional content; good sensory acceptability D = Not reported in the study [66] Nutritionally enhanced wheat flour cookies with tamarind seed flour A = Increased protein content and antioxidant properties D = Not reported in the study [67] Cookies formulated with date seed powder A = Improvement in sensory characteristics and nutritional profile; can be used as a partial substitute for wheat flour D = Not reported in the study [68] Avocado stone flour as a mixture of cassava flour in the manufacture of cookies A = Acceptable sensory characteristics; high antioxidant activity D = Not reported in the study [69] Addition of guavira flour in breads A = Increased vitamin C content; good sensory quality.D = Not reported in the study [52] Preparation of cookie with passion fruit peel flour Technological potential for the bakery market.D = Not reported in the study [70] Functional bread made with mango stone flour A = Good nutritional characteristics; antioxidant potential.D = Not reported in the study [71] Partial substitution of wheat flour by prickly pear stone four in breads.
A = Increased content of total phenolics, flavonoids and radical scavenging activity; increased dietary fiber, fat and ash content; reduction in carbohydrate content.D = Not reported in the study [30] Cake fortified with pomegranate seed powder.
A = Increase in protein, fat and fiber content of the product; up to 5% replacement of wheat promotes nutritional enrichment without affecting the quality and acceptance of the cake.D = Not reported in the study [72] Effect of grape seed flour on antioxidant profile, textural and sensory properties of waffles A = Improved total phenolic content and antioxidant activity of the final product D = Not reported in the study [73] Passion fruit mesocarp flour (PFMF) in pasta on the quality of dry noodles A = Considerable increase in fiber content D = Substitution of PFMF hindered the formation of the gluten network; the quality of dry noodles was affected by PFMF substitution. [74] Integral bread rolls with the addition of jaboticaba peel flour A = Increased fiber content; increased content of phenolic compounds; increased antioxidant activity.D = Not reported in the study [75] Integral bread enriched with papaya by-product flour A = Increased fiber and protein content D = Decrease in the technological quality of the final product [42] Composition and sensory acceptability of muffin with jaboticaba peel flour A = Reduction of lipids in products; increase in fiber content; good acceptability by children with the addition of up to 9% of jaboticaba peel flour lipids in products D = Decrease in protein content [76] Substitution of wheat flour by jackfruit seed flour in fried donuts A = Increasing the amount of fiber and protein D = Hardness, resilience, chewiness, and firmness were negatively affected due to the difficulty of gluten formation. [77] Red mombin seed flour as a functional component in chocolate brownies A = Improved nutritional content, especially in relation to dietary fiber and mineral content.Improved elasticity, cohesiveness, gumminess, chewiness, and resilience.D = Not reported in the study [78] Legend: (A) advantages; (D) disadvantages.
As can be seen, products made with flours obtained from fruit peels and seeds, when added to formulations, can improve the quality while maintaining acceptable sensory characteristics; it is necessary to emphasize that the complexity of the fruit co-product matrix can promote different interactions between the components and aspects related to particle size, water retention capacity, gel formation capacity, among others, which can affect to a lesser or greater extent the physicochemical, technological, nutritional, and sensory profiles of the final product, generating different responses [18].The technological characteristics, for example, can be influenced due to the amount of fibers that are generally present in these types of flours; however, the cost-benefit of this addition in adequate proportions can be positive, since, in relation to the health benefits of the consumer, the presence of this constituent can contribute to a final product of higher quality than its traditional version; however, this evaluation must be performed carefully and respect the characteristics of each product, which requires in-depth studies of these formulations.It is interesting to note that bakery products are one of the most consumed food groups worldwide, with refined wheat flour being the main ingredient.However, refined flour has a high glycemic index and despite its excellent technological characteristics for bakery products, its use as an ingredient imposes restrictions on individuals suffering from pathologies associated with its intake [79,80].This makes the inclusion of fruit co-product flours in bakery products, such as breads, cakes, and cookies, an interesting alternative for improving the nutritional and functional profile of these products [71,81].
Studies show that it is feasible to partially replace wheat flour with fruit co-product flour in various preparations and that this change in formulation is associated with a positive effect on the total phenolic content, antioxidant capacity, fiber, and mineral content, suggesting a potential for making functional products [30,72,82].The incorporation of flours obtained from unconventional sources, used as ingredients in new food formulations, can help to modulate physiological responses associated with an immense range of compounds derived from the secondary metabolism of vegetables and which remain present even after processing and making the product/preparation.These flours obtained from fruit peels and seeds have been consistently reported to reduce the risk of chronic diseases, due to the presence of compounds with biological effects that act to protect against DNA damage and human LDL oxidation, act to control body weight gain, and act to modify the absorption of glucose and triacylglycerol by inhibiting the activities of α-glucosidase and lipase, respectively [64,83,84].In addition, the consumption of dietary fiber easily found in these types of flour can also contribute to a lower risk of developing and worsening gastrointestinal diseases and other diseases associated with metabolic disorders, such as diabetes, dyslipidemia, and cardiovascular diseases [64,85].
Another key point for the inclusion of fruit co-product flours in human diets is the idea that they can contribute to achieving more adequate nutrition, especially among the most vulnerable populations [86].The prevalence of hunger and malnutrition remains alarming, as do other pathologies aggravated by nutritional deficiencies caused by monotonous, low-quality diets [87].As a result, the use of these flours as food complements, where they can be added directly to meals to provide additional loads of specific vitamins or minerals in order to reverse or help recover imbalances in the levels of certain nutrients, or as ingredients in the formulation of other food products as a way of improving the nutritional content offered, is considered an emerging strategy that complements other traditional strategies against malnutrition.Food insecurity in the face of climate change is a reality and requires measures that can guarantee the quality and availability of food that can meet the nutritional demands required by each age group with specific needs [88].To obtain and target these products correctly, complete analyses of supply chains and their interfaces with food and nutritional security are needed to provide additive solutions, including waste minimization, the use of co-products, and the implementation of new technologies to complement traditional industries with the aim of fostering accessible nutrition for the world [89].
This scenario shows that, by including these co-products in the diet, it is possible to mitigate the effects of the food deficit, since these fractions contain minerals, vitamins, fatty acids, and fibers, as well as energy content capable of contributing to meeting daily caloric needs.These flours, which in most cases have a low production cost and an abundance of raw materials, can be a viable alternative, especially in countries where the problems of malnutrition and hunger exist at a higher level.Although hunger and malnutrition are a global problem, it is in the least developed countries that the population is most affected.In this sense, directing the high volume of waste generated in these production centers towards the transformation of products with added nutritional value will have direct benefits for local communities [90,91].
Thus, in addition to the health benefits, the use of fruit co-products opens new perspectives for the food industry in terms of new ingredients, which, in addition to aspects related to new processing technologies for the sector, contribute to better waste management, food and nutrition safety strategies, and sustainability actions (Figure 2).Encouraging a culture of valuing co-products on a small, medium, or large scale reaffirms the concept that resilience and sustainability are inseparable and that a resilient food system can provide sufficient, adequate, and accessible food for all, even in the face of various and even unforeseen disturbances [34].In addition, we have seen that the consolidation of new knowledge and opportunities will be reaffirmed as new ecological practices, essential in the efficient use of resources, are applied in the development of products, new business models, and innovative attitudes.

Conclusions
The fruit agro-industry generates the largest volume of waste among the food sectors, which demonstrates the need to develop strategies capable of promoting the transformation of these fractions into products suitable for human consumption and reducing damage to the environment.Flours are the most versatile and simple form for incorporation into other food products, offering an extensive portfolio of applications.However, many challenges need to be overcome, requiring in-depth studies on drying techniques to reconcile process optimization, on mitigation of nutritional and bioactive compound

Conclusions
The fruit agro-industry generates the largest volume of waste among the food sectors, which demonstrates the need to develop strategies capable of promoting the transformation of these fractions into products suitable for human consumption and reducing damage to the environment.Flours are the most versatile and simple form for incorporation into other food products, offering an extensive portfolio of applications.However, many challenges need to be overcome, requiring in-depth studies on drying techniques to reconcile process optimization, on mitigation of nutritional and bioactive compound losses, and on the application to an industrial scale.Studies are also needed to reduce sensory and rheological impacts to improve the acceptability of these products on the market.In addition, it is important to remember that the use of shell and seed flours for human nutritional purposes must meet criteria that go beyond their ability to be incorporated into formulas, requiring a standard of identity, quality, and safety, which points to a long way to go in research into the use of co-products.
In view of this, we can point to the promising outlook for the application of fruit byproduct flours in the development of new products or to add value to traditional products.In addition, this strategy can be seen as an important point among actions to reduce losses in supply chains and food and nutritional security.Flours from co-products are emerging as low-cost alternatives, capable of making up diets that provide a better supply of nutrients and energy to populations plagued by food insecurity.
Therefore, the breadth of discussions on the valorization of fruit by-products in human nutrition allows us to reflect on many issues, including the role of the main agents that can accelerate or mitigate measures that impact sustainable practices.Finally, even though the modern food industry currently occupies a decisive position in the efficient management of waste prospects for conversion into co-products, a closer approach between research institutions, government, producers, and other food sector participants is necessary to ensure that the use of these raw materials can be utilized in a way that effectively contributes to sustainable practices.Therefore, there seems to be a promising inter-and transdisciplinary scenario in the search for a more sustainable model for survival and, above all, for caring for this and the next generations.

16 Figure 2 .
Figure 2. Conversion cycle of fruit waste into co-products in the perspective of sustainable strategies.

Figure 2 .
Figure 2. Conversion cycle of fruit waste into co-products in the perspective of sustainable strategies.

Table 1 .
Use of drying applied to fruit co-products to obtain flours.